Multiphase fluids refer to compositions of one or more materials, each potentially in different states of solid, liquid and gas and potentially in partially dissolved or immiscible states, which are also referred to as multiphase flows when flowing through a system. Multiphase flows/fluids are widely present in many processes ranging from food to petrochemical industries. Multiphase composition metering is an emerging technology that has a high potential, especially in the oil and gas industry.
Traditionally, the multiphase composition measurement in the oil and gas industry is conducted periodically (often based on regulatory requirements), on the scale of days or weeks. This measurement is performed by selecting and routing a single well stream from the production field into a test separator that separates the multiphase flow into single phase streams, and it is these single phase streams which are then analyzed. This long-standing practice has been the standard for decades but has become increasingly prohibitive in optimized production allocation systems that must react in real-time to changes in well stream compositions across the production field. As a result, there is a need for “real-time” multiphase meters which are capable of measuring multiphase composition either within the flow or at least without the need to disrupt the streams.
There have been some developments in this field. Framo Engineering developed and field tested a multiphase meter targeted for subsea installations that employed gamma-ray attenuation sensing and a cross-correlation to calculate phase fractions. However, the requirement to pre-mix the input stream leads to a less robust solution and less useful production output. Multiphase meters must reliably operate under a range of unpredictable multiphase flow conditions such as plug or slug flow. Existing multiphase meters, including the Framo meter, often use a mixer to yield a homogenous mixture essential for the accurate measurement. However, mixing complicates any downstream separation process, as fine multiphase mixtures prove difficult to separate without using additional chemicals.
Framo Engineering together with Schlumberger have also developed a meter that employs high-frequency gamma-scanning at different power levels to mitigate the criticality of mixing. But the system remains complex.
There remains a desire for a multiphase meter which has a simplified design and which can determine the composition of a production stream of fluid in an industrial process. References to oil/gas flow are for convenience, and are not meant to limit the applicability of this invention to other industrial processes involving suitable mixed composition flows.
There is a desire for such a system which does not require a nuclear particle emitter and detector or unnecessary mixing of the composition, but which may also be compatible with such a system.
Various types of probes are currently used in multiphase analysis. For example, capacitive sensors robustly operate in many industrial sensing applications such as level measurement, proximity/position sensing, product quality detection (synthetic yarn) and many others. The capacitive or electrostatic technique has been applied to multiphase flow measurement but existing applications do not effectively address wide varieties of multiphase flow regimes.
In 1988, Van der Linden, “Capacitive Sensors: Design and Applications”, Wiley-IEEE Press (1996), pp. 133 (Ch. 9.5 Water/Oil Mixture Probe), ISBN-13: 978-0780353510 (“Van der Linden”) developed an oil-water percentage probe for water contaminated oil streams. The method employed an AC bridge with a temperature compensated oil dielectric reference. The signal conditioning eliminated the effects of parasitic capacitance by using low-impedance amplifiers and the probe featured good accuracy. However, the system was not applicable to a wide range of oil-water percentages, the mixture needed to be homogenous, and drifting of the dielectric constant beyond the temperature linked drifts from the oil dielectric reference cannot be detected by this method. The presence of an externally powered pump that circulates the oil reference adds system complexity and power inefficiency.
A variety of capacitance sensors for determining volume fractions in two-phase pipelines were researched by Abouelwafa and Kendall, “The Use of Capacitance Sensors for Phase Percentage Determination in Multiphase Pipelines”, Abouelwafa, M. Sami A.; Kendall, E. John M.; Instrumentation and Measurement, IEEE Transactions on Volume 29, Issue 1, March 1980 Page(s): 24-27 (“Abouelwafa et al.”).
In Abouelwafa et al., concave plate electrodes affixed internally and externally to a pipe were compared in sensing a fluid's permittivity and conductivity. A measurement of complex impedance, or simply conductance and capacitance has also been examined as a possible proxy for determining multiphase composition ratios. However, this research did not address changes in the properties of the flow, and there remains a need to develop a sensor capable of multiphase measurement where phase properties or temperatures of the materials drift.
Multiphase flow measurement requires the knowledge of flow rates of individual phases or the total multiphase flow rate in m3/sec or gal/min and the composition. The composition is defined as the volumetric ratio of individual phases against the total tested volume. In the Oil & Gas industry for example, the best case scenario is pumping 100% hydrocarbons and 0% water, which is rarely the case. However, the worst case scenario of pumping 100% ‘produced water’ is also rare. Therefore, ideally, the composition ranging from 0% to 100% must be considered. Note that the extreme 0%/100% composition actually constitutes a single phase flow.
Additional problems exist in the prior art in relation to: measuring all possible composition ratios of a multiphase fluid, i.e. 0% to 100% of all fluids; and where a homogeneous mixture is not required, reliable stratification of the flow for the purposes of measurement. The prior art identifies horizontal stratification pipes, which are unreliable in high flow rate applications, or large-size expensive separators used to slow down the stream significantly. Where one phase is conductive, gravity based stratification may be amplified using an electromagnetic field, but the large power consumption, complexity and, in the case of combustible fluids, danger, in this approach make it highly impractical.
For known horizontal stratification pipes of rectangular cross section, whether they operate as parallel capacitors or series capacitors, linearly independent measurements of the flow cannot be obtained by changing the aspect ratios of the pipes, only by selectively omitting to measure part of the flow. Since additional measurements are needed for each additional parameter (unknown) of the flow, it quickly becomes difficult to solve for more than two variables without greatly restricting the composition fractions which can be measured.
There is a need for a multiphase sensor which is able to operate in the full measurement range, with improved stratification of the phase layers, and which is capable of operating with a variety of metrics.